Characterization of local multi-physics phenomena in the CABRI research reactor

The IRESNE R&D institute at CEA Cadarache invites applications for a post-doctoral position whose aim is first to develop a coupling between the APOLLO3®/THEDI core model at the pin-scale and the CATHARE model of the 3He depressurization system. Then, with the support of this simulation tool, the second objective will be to define core configurations of interest and measurements for characterizing local multi-physics phenomena in CABRI.
The CABRI pool-type research reactor located at CEA Cadarache is dedicated to the analysis of nuclear fuel behavior during Reactivity-Injection Accident (RIA) in Pressurized Water Reactors. The reactor experimentally simulates power pulse transients in the driver zone, which induces a RIA-representative energy deposition in the fuel sample of a water-loop at the core center. The power transients in the CABRI core are initiated by the depressurization of transient rods containing a strong neutron absorber, 3He.
Two models have been recently developed to simulate CABRI power transients. The first model at the assembly scale is a tool called PALANTIR, based on the CATHARE2 system thermal-hydraulics code with additional surrogate models to take into account the reactivity injected by 3He depressurization. The CATHARE2 code includes a neutron point kinetics module, a heat equation solver module and simplified thermomechanical models for fuel pins. In addition to the core, the depressurization circuit is modeled, providing access to 3He density in the transient rods.
The second model, at the pin-scale level, is based on a APOLLO3®/THEDI coupling via the C3PO platform. APOLLO3® solves the simplified transport equation. THEDI is used to model an unsteady 1D two-phase hydraulics flow in the core. It also solves the 1D heat equation for fuel thermics. For the simulation of each power transient in CABRI, PALANTIR provides the 3He density evolution versus time; these data are imposed as boundary condition in the APOLLO3®/THEDI coupling.

Deep learning methods with Bayesian-based uncertainty quantification for the emulation of CPU-expensive numerical simulators

In the context of uncertainty propagation in numerical simulations, substitute mathematical models, called metamodels or emulators are used to replace a physico-numerical model by a statistical (or machine) learning model. This metamodel is trained on a set of available simulations of the model and mainly relies on machine learning (ML) algorithms. Among the usual ML methods, Gaussian process (GP) metamodels have attracted much interest since they propose both a prediction and an uncertainty for the output, which is very appealing in a context of safety studies or risk assessments. However, these GP metamodels have limitations, especially in the case of very irregular models. The objective of the post-doctorate will be to study the applicability and potential of Bayesian-based deep learning approaches to overcome these limitations. The work will be focused on Bayesian neural networks and deep GP and will consist in studying their tractability on medium size samples, evaluate their benefit compared to shallow GP, and assess the reliability of the uncertainty associated with their predictions.

Innovative strategies for minor actinides using molten salt reactors

Within the framework of the ISAC (Innovative System for Actinides Conversion) project of the France Relance initiative, preliminary concepts of molten salt reactor capable of incinerating minor actinides have to be proposed in connection with prospective évolutions of the French nuclear fleet (stabilisation or reduction of the plutonium and americium inventory, minimization of the deep storage footprint, …) and contraints linked to the nuclear fuel cycle (plutonium and minor actinides inventories). The specificities of molten salt reactors will be exploited to design innovative transmutation strategies.
The postdoctoral fellow will be based in the reactor and fuel cycle physics unit of the IRESNE R&D institute at CEA Cadarache. He/she will develop expertise in neutronics, fuel physics, and in the design of Generation-IV reactors of the molten salt type.

Design of innovative nuclear systems cooled by heat pipes

The combined goals of CO2 emission reduction and energy self-sufficiency, in the current geopolitical context, open up new perspectives for nuclear applications (cogenerations, hydrogen production, etc.). In particular, the MNR concepts (Micro Nuclear Reactors), with a thermal power of 2 to 50 MW, bear the promise of flexibility, while providing much reliability and accrued safety.
Among the MNR technologies, the particular concept in which the core is cooled by heat pipes strongly improves the inherent safety of the design, in normal and in accidental conditions as well.
In order to demonstrate the feasibility of such an MNR technology, a predesign of a single high temperature heat pipe should be performed for different selected technologies. Then, the overall heat pipe cooling system should be evaluated. Finally, after having modelled the core cooling system, an integration study including a predesign of the core itself should be done with the two subsystems coupled.

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